A wristband antenna for a radio-electric receiver integrated in a watch is known from in the prior art, particularly from EP Patent No. 0 308 935. As is shown schematically in FIGS. 1A and 1B, watch 1 is formed of a case 2 containing radio-electric receiver 3 and a wristband 4. The back cover of the case includes a first metal part 5, which forms earth for receiver 3. The radiating element of the antenna is formed by a first metal conductor 6 arranged inside wristband 4 and connected to the input of radio-electric receiver 3. This first conductor 6 has the shape of a sinuous wire between the two side walls of wristband 4 whose unfolded length is close to λ/4, λ being the wavelength corresponding to the central frequency of the signal to be received. The wristband antenna can also include a second conductor 7 arranged in the wristband and connected to the receiver's earth. This conductor 7 can have the form of a film. Each of the two conductors 6 and 7 is respectively arranged in one of the two strands 8 and 9 of wristband 4.
This wristband antenna behaves like a dipole antenna with an electric length equal to λ2. When watch 1 is secured to the user's wrist, back cover 5 of the case, or the second strand 9 of the wristband, ensure that receiver 3 is earthed. The other strand 8 of the wristband forms a capacitor picking up the radial field around the arm. The dipole is formed, at least in part, by the sinuous structure of first conductor 6 allowing integration of the tuning inducting coil of the antenna in the strand itself. It is also possible to use a metal bezel 10, shown solely in FIG. 1B, insulated from back cover 5 of the case, to increase the effective mean height of the wristband antenna.
The electric connection between conductor elements 6 and 7 arranged in strands 8 and 9 of the wristband and receiver 3 arranged in case 2 can be achieved by means of electric contacts via bars, flexible connections by film or crimping, these elements not being shown in FIGS. 1A and 1B. The insulation between the receiver's earth and the radiating element forming the dipole antenna is assured by the body of case 2, which is consequently necessarily made with an insulating material.
Within the scope of the present invention, it has been demonstrated that such a dipole antenna integrated in wristband 4 of a watch has certain drawbacks. First of all, the electric connections between conductor elements 6 and 7 arranged in strands 8 and 9 of the wristband and receiver 3 have complications if one wishes to seal the interior of case 2. One solution, which would consist in forming these connections respectively via back cover 5 of the case and via bezel 10, both metallic, requires, however, either passing through insulating case 2, which makes manufacturing considerably more complex and expensive, or leaving electric conductors visible, which is undesirable, particularly from an aesthetic point of view. It will also be noted that such a dipole antenna integrated in the wristband requires the use, at least for the radiating element, of a conductor 6 of sinuous shape with a determined unfolded length dependant on the desired wavelength corresponding to the central frequency of the signal to be received, which may, depending upon the desired reception frequency, raise problems regarding the robustness of the antenna structure and thus operating reliability over time.